# HOW TO USE SURFACE INDEX TO FIND PROPORTION OF FINE AGGREGATE TO COARSE AGGREGATE?

|Our aim is to mix the fine aggregate & coarse aggregate in such a ratio so that the final combined aggregate gives the specified or desirable surface index. Follow the below mentioned steps to find the proportion of fine to coarse aggregate.

**Procedure**

- Calculate the total surface index of fine aggregate (let it be
**x**) - Calculate the total surface index of coarse aggregate (let it be
**y**) - Let
**z**be the total surface index of combined aggregate and**a**is the proportion of fine to coarse aggregate. - Then
**a = (z-y)/(x-z)**

**Worked out Example**

** Calculating total surface index of fine aggregate**

Sieve sizes within which particles lie | Percentage of particles within sieve size | Surface index for particles for within sieve size | Calculation of total surface index |

4.75 mm to 2.36 mm | 10 | 4 | 40 |

2.36 mm to 1.18 mm | 20 | 7 | 140 |

1.18 mm to 600 micron | 20 | 9 | 180 |

600 micron to 300 micron | 30 | 9 | 270 |

300 micron to 150 micron | 15 | 7 | 105 |

Total = | 735 | ||

Adding constant of 330 | 1065 |

Total surface index of coarse aggregate (i.e. **x**) = 1065/1000 = 1.065

**Calculating total surface index of coarse aggregate**

Sieve sizes within which particles lie | Percentage of particles within sieve size | Surface index for particles for within sieve size | Calculation of total surface index |

20 mm to 10 mm | 65 | -1 | -65 |

10 mm to 4.75 mm | 35 | 1 | 35 |

Total = | -30 | ||

Adding a constant of 330 | 300 |

Total surface index of coarse aggregate (i.e. **y**) = 300/1000 = 0.30

Let the total surface index of combined aggregate (i.e. **z**) = 0.60

So **a = (z-y)/(x-z)**

**=** (0.60-0.30)/(1.065-0.60)

= 1/1.55

So **fine** **aggregate : Coarse aggregate = 1 : 1.55**